Research

Fig. 1: Musculoskeletal biomechanics (MSB) research team combining experimental, imaging, computational and clinical studies.

 

Imaging of single mineralised collagen fibrils

Postdoc grant UniBE with LBN

Bone is a complex material that offers a unique blend of strength, toughness, and lightness. The key feature contributing to this outstanding combination of material properties is the hierarchical arrangement of bone spanning across the length scale.

Fig. 2: TEM image of a mineralised collagen fibre, with a zoom-in view of the mineral crystal.

At the lowest level, bone is built up of mineralized collagen fibrils (MCFs). In this highly collaborative project, we used advanced transmission electron microscopy (TEM) techniques to assess the structural organization of the MCFs. We used TEM to visualize the arrangement of collagen molecules and minerals within the MCFs at the nanoscale. We also measured the orientation of individual mineral crystals within the MCFs. The capabilities of the TEM allow us to resolve MCF organization and composition down to the nanoscale and even Angstrom level, opening-up new venues for research into how bone diseases and treatments affect the ultrastructure of bone.

 

Contribution of Bone Tissue Properties to Strength of the Ageing Human Hip

SNF grant # 200365 with EMPA, PSI, VUT & MUG
This project investigates structure-property relations in the femoral neck cortex and their change with age.  Ninety-four femurs were analyzed through histology and small-angle x-ray scattering (SAXS) for micro- and nano-structural analysis, respectively. Selected samples underwent in-depth  investigation through micropillar compression and 3D structural analysis by small- and wide-angle x-ray scattering tensor tomography (SASTT and WASTT). Histological analysis suggested a steady microstructure with age and a SWASTT reconstruction is shown in Fig. 3.

Fig. 3: SASTT and WASTT reconstructions of a lamellar bone micropillar, showing the ratio of scattering intensity and the main structural orientations.

Besides experiments, an elasto-plastic constitutive model of bone tissue was extended to account for its viscoelastic behaviour at multiple time scales. This model was validated with experiments at different strain rates, including quasi-static and drop-tower tests.

 

HR-pQCT-Based Diagnosis of Osteoporosis

with IS & MG
The motivation driving this project is the prevention of osteoporosis. The objective is to develop a diagnostic method that makes use of high-resolution peripheral quantitative computed tomography (HR-pQCT). In vivo imaging of the distal radius and tibia allows the generation of non-linear homogenised finite element (hFE) models that are capable of accurately predicting stiffness and strength.

Fig. 4: Distribution of damage on an HR-pQCT-based hFE mesh of the distal tibia following uniaxial compression.

The introduction of a novel meshing technique using a smooth, structured, hexahedral mesh improves both mapping of material properties and computational efficiency. By creating smooth meshes, the complex geometry of the thin cortical bone is preserved, resulting in better predictions of strain fields. By creating structured meshes with consistent element to element correspondence, a comparison between patient models or in longitudinal settings, becomes accessible and provides a supplementary clinical information.

 

A Fragility Fracture Integrative Risk Model for CT Recycling

SNF grant # 183584 with HUG, IS, & MUG
Experimental results characterising the properties of soft tissues covering the hip were exploited to develop a 1D impact model for the sideways impact of the human hip resulting from a fall from standing height. The developed impact model was combined with a previous fall risk model and a computer tomography (CT)-based FE method for bone strength estimation, constituting an integrative approach to hip fracture risk.

Fig. 5: A) Impact force resulting from sideways fall. B) Sensitivity of fracture risk to variations in femoral strength and other anatomical quantities.

Data collected in the parallel clinical study over the past years was used to perform a sensitivity analysis of the mechanistic-stochastic fracture risk model. In addition, the relations between trochanteric soft tissue thickness and clinical variables, such as BMI, were explored.  CT-like images constructed using statistical shape models from standard DXA were analysed to examine alternative inputs for the fracture risk model where CT images are not available.

 

Biomechanical Stability of Dental and Orthopaedic Implants

Innosuisse grant # 115.975 with ZMK, AO & industry partners
Primary stability (PS) is a key factor for promoting osseointegration and long-term success of dental implants,  particularly for immediate loading scenarios. Implant size and design, insertion protocol and especially bone quality are major factors influencing PS. Current methods for assessing PS are either per- or postoperative and an objective measure of PS is therefore missing in surgical planning today.

Fig. 6: Process illustration starting from the CBCT image to bone density assessment, virtual implant placement and finite element simulation.

The development of a potential solution to bridge this gap was undertaken that consists in combining a calibrated cone beam computer tomography (CBCT) reconstruction with a non-linear finite element analysis to predict PS for different protocols before surgery. Similarly, primary implant stability is crucial for successful total hip arthroplasty. Thus, geometric characteristics of the human proximal femoral medullary canal were investigated in relation to sex and age and were captured in a statistical shape model.